There is a very large, and still rapidly growing, demand for wireless communication services today. Mobile telephone services are used to a very large extent, e.g. for telephone conversations and text messages. Also, high speed wireless communication is used for a large number of applications, such as internet browsing, streaming of music, films and/or television, and the like.
Since the demand for wireless communication services is very high, and the end users expect the wireless services to be available essentially everywhere they go, the wireless communication system coverage must cover very large geographical areas, both indoors and outdoors, and also provide high bitrates in these areas. Traditionally, radio base stations, such as Base Transceiver Stations (BTS) and eNodeBs, typically being located close to a tower comprising one or more tower-mounted antennas, are provided with all the circuitry necessary for sending and receiving the wireless communication signals to and from the mobile equipments connecting to it, such as Mobile Stations (MS) or User Equipments (UEs). The radio base stations have typically been arranged in cells. The radio base stations include both the circuitry to receive and transmit signals from and to a core network and to receive and transmit signals from and to the one or more antennas, including radio frequency (RF) circuits and power amplifiers. The mobile equipment also include the circuitry for receiving and transmitting signals from and to the radio base stations, including radio frequency (RF) circuits and power amplifiers.
Also, the end users nowadays expect high speed wireless communication services, such as mobile telecommunication services, wireless fidelity networks (WiFi), and Wireless Local Area Networks (WLANs), to be available essentially everywhere. Therefore, a large number of different types of radio transmitting units are distributed in indoor and outdoor locations, such as in malls, railway tunnels, road tunnels, restaurants, cafés, airports, conference centers, tunnels, stadiums and exhibition halls.
Distributed systems have been developed for providing coverage and high bitrates in locations for which the traditional radio base station concept results in poor service coverage and bitrates. One example of such a distributed system is a Distributed Antenna System (DAS). In a DAS one or more Remote Units (RUs), each possibly being utilized by one operator/service provider and providing one system and/or service, are connected to one or more Remote Unit Controllers (RUCs) and/or one or more fiber cables. Typically, the one or more RUCs are radio base stations, such as a BTS or an eNodeB or the like. These radio base stations provide RF signals related to one or more services and/or service providers to the RUs via fiber cables. The RUs transmit/receive the RF signals to/from the mobile equipment.
Also, smaller cells have been developed, such as pico cells and femto cells, which can be used for increasing coverage and bitrates, and to lower the costs. The micro base stations and pico base stations are complete standalone radio base stations, including all the circuitry of the traditional radio base stations, including all the circuitry necessary for sending and receiving the wireless communication signals to and from the mobile equipment, including power amplifiers. The pico cells and femto cells can be deployed such that coverage and bitrates can be optimized for the geographical area of the communication system, both outdoors and indoors.
A further development of the smaller cell concept is the Remote Radio Head (RRH) concept. The RRH concept breaks up the traditional radio base station architecture into a possibly centrally located processing facility, a so called RRH controller, and one or more distributed antennas units, in this document called RRH units, being connected to the processing facility through a network preferably having a high bandwidth. Here, all the traditional radio base station processing equipment except for the radio frequency processing equipment and the power amplification equipment are located in the RRH controller, whereas the radio frequency processing equipment and power amplification equipment are located in the distributed RRH units.
As has been described above, there are today a number of concepts available for providing and extending the coverage of mobile services and for enhancing the bitrates at certain locations in the systems. There are also a number of other such concepts available for extending the coverage and/or enhancing the bitrates. In all of these concepts, a radio signal is transmitted between a radio base station/RU/RRH and a mobile equipment of some kind over an air interface. This transmission over the air interface requires for all of these concepts for an amplification of the signals to be transmitted. This amplification is typically performed by utilization of one or more power amplifiers.
Power amplifiers are often utilized as the last amplifier in a transmission chain, i.e. at the output of the transmission chain. The power amplifiers are thus responsible for providing an RF output signal having a sufficient signal power to an antenna arrangement. Generally, the signal power should be high enough to provide a predetermined transmission coverage and/or error rate for the radio transmission. However, the signal power, and also other features of the transmitted signals, must be kept within predetermined limits, in order to reduce interference in the radio transmission system into which the radio signal is transmitted.
There are a number of different classes of amplifiers that are used for power amplification, such as classes A, B, AB, C, D, E, and additional classes, e.g. Doherty amplifiers.
Class A amplifiers utilize the whole input signal for the amplification and the conductive element in the amplifier conducts during the whole amplification cycle time. Class A amplifiers are simple and add relatively little distortion to the signal.
Class B amplifiers only utilize half of the input signal for the amplification and the conductive element in the amplifier conducts during half of the amplification cycle time. Class B amplifiers are power efficient but add a relatively large amount of distortion and/or non-linearity to the signal.
Class AB power amplifiers can be seen as a compromise between class A and class B amplifiers. The class AB amplifier generally operates the same way as the class B amplifier over half the amplification cycle time, but also conducts a small part on the other half of the cycle. Class AB amplifiers thereby sacrifice some power efficiency over class B amplifiers, but gains in linearity. Class AB amplifiers are typically much more power efficient than class A amplifiers.
Class C amplifiers conduct less than half of the input signal and the distortion at the output is high. However, high power efficiencies are possible. A common application for class C amplifiers is in RF transmitting devices operating at a single fixed carrier frequency, where the distortion is controlled by a tuned load of the amplifier. The input signal is used to switch the active device, which causes pulses of current to flow through a tuned circuit being a part of the load. The tuned circuit resonates at one frequency, e.g. at the fixed carrier frequency, which suppresses the unwanted frequencies, and the wanted full signal can be extracted by the tuned load.
A Doherty amplifier is a hybrid configuration including a primary/carrier stage in parallel with an auxiliary/peak stage. The input signal is split in order to drive the two amplifying stages, i.e. the primary/carrier stage and the auxiliary/peak stage. A combining network sums the output signals from the two amplifying stages and phase shifting networks are used at the inputs and outputs of the Doherty amplifier. During periods of low signal level, the primary/carrier stage efficiently operates on the signal and the auxiliary/peak stage is cutoff, whereby the amplifier consumes little power. During periods of high signal level, the primary/carrier stage delivers its maximum power and the auxiliary/peak stage delivers up to its maximum power. The primary/carrier stage can be implemented e.g. by use of a class B amplifier or by a class C amplifier. The auxiliary/peak stage can be implemented e.g. by use of a class C amplifier.
Usage of power amplification in communication systems for amplification of the RF signals, which is necessary in order to provide sufficient coverage and bitrates, has a problem in lack of linearity of the power amplifiers. Non-linearity of the power amplifiers prevents the power amplifiers to accurately reproduce the signal being applied to its input port. Also, non-linear power amplifiers used in a transmitter in a communication system cause the transmitted signal to leak into adjacent frequency radio frequencies/channels, which creates distortion in these adjacent radio frequencies/channels.
The non-linearity of the power amplifiers limits the performance of the whole communication system. Non-linearities in the frequency response of the power amplifiers create distortions that also limit the dynamic range of the amplifiers. Also, the created distortions degrade the overall system performance since the distortions are transmitted over the air interface of the communication system, whereby other signals and/or entities in the communication system can be distorted.
A large number of solutions, e.g. utilizing predistortion, have previously been presented in order to solve these problems. When predistortion is utilized, an inverse model of the assumed amplifier distortion is introduced into the amplifier input. The idea is that the predistortion should cancel out the distortion such that the predistortion and the distortion together result in an amplifier circuit having linear gain and phase. However, such previously known solutions tend to decrease the amplifier efficiency while maintaining a high current consumption. Also, the previously known solutions increase the hardware complexity and cost considerably.